1. Field of the Invention
The present invention is concerned with a method for developing electric latent images, and more particularly relates to a developing method in which electric latent images formed according to the electrophotographic process, electrostatic recording process and the like are developed with an insulating toner electrically charged by friction, without use of carrier particle, to obtain visible images.
2. Description of the Prior Art
Various electrophotographic processes are heretofore known as disclosed, for example in the specifications of U.S. Pat. No. 2,297,691 and British Pat. Nos. 1,165,406 and 1,165,405. In these processes, generally, electric latent images are formed on a photosensitive member utilizing a photoconductive material by various kinds of means and ways, and developed with a toner. Further, the resulting toner images are, if necessary, transferred onto an image transfer material such as paper or the like and ultimately fixed by application of heat, pressure or solvent vapor to obtain reproductions.
Methods for visualizing electric latent images with a toner are also known, including the magnetic brush method disclosed in U.S. Pat. No. 2,874,063, the cascade developing method disclosed in U.S. Pat. No. 2,618,552, the powder cloud method disclosed in U.S. Pat. No. 2,221,776, the fur brush method, liquid developing method and other various methods. Among these methods, the magnetic brush method, cascade method, liquid developing method and the like which use a developer composed mainly of a toner and carrier are particularly put into practical use in the wide field. Although these methods are excellent in providing good images in a relatively stable state, they have common drawbacks resulting from use of the two-components type developer as mentioned above, such as deterioration of the carrier and variation in the mixing ratio of the toner and carrier.
Therefore, for the purpose of avoiding such drawbacks, various developing methods are proposed, in which a one-component type developer composed only of a toner is employed. For example, a method of effecting the development with a magnetic toner having electric conductivity is disclosed in U.S. Pat. No. 3,909,258, in which method the electrically conductive, magnetic developer is supported on an electrically conductive sleeve in a cylindrical shape provided with magnetism in the inside and then brought into contact with electrostatic images to carry out the development. More particularly, the development is effected in such a manner that an electric conduction path is formed through the toner particles in the development portion between the surface of the recording material and that of the sleeve, and electric charges are led through the conduction path from the sleeve to the toner particles, and further the toner particles are then caused to adhere onto the image portion of the electrostatic images by means of the coulomb force produced between the toner particles and the image portion of the electrostatic images.
Such developing method using the electrically conductive magnetic toner is able to avoid the problems involved in the conventional methods utilizing the two-components type developer and therefore is an excellent method. However, it is inadvantageous in that since the toner is electrically conductive, the developed image is difficult to transfer electrostatically from the photosensitive member to the final supporting material such as plain paper or the like.
As for a developing method using a high resistant magnetic toner capable of being electrostatically transferred, Japanese Patent Laid Open No. 52-94140 discloses a method utilizing the dielectric polarizaton of the toner particles. Such method, however, has drawbacks that the developing speed is slow and the density of the developed image is insufficient. Therefore, many difficulties for practical use are involved in that method.
Another developing method using a high resistant magnetic toner as heretofore known is such one that the toner particles are subjected to triboelectrification, for example, by mutual friction of the toner particles and friction between the toner particles and sleeve surface, and then brought into contact with an electrostatic image-holding member to effect the development. However, this method is inadvantageous in that the triboelectrification of the toner particles is liable to become insufficient because of few frequency in contact between the toner particles and rubbing element, and the electrically charged toner particles is apt to agglomerate on the sleeve owing to the intensified coulomb force between the electrically charged toner particles and the sleeve. Therefore, also in this method, there are many difficulties for practical use.
The Applicant has previously proposed a novel developing method capable of eliminating the above-mentioned drawbacks and inadvantages, as disclosed in Japanese Patent Laid Open No. 54-43036. According to this method, a magnetic toner is very thinly coated to a sleeve and given charges by the triboelectrification. The charged toner is brought very close to an electrostatic image, but not in contact with the image, under the action of magnetic field so that they are facing each other, and consequently the development is carried out.
This developing method is able to obtain excellent images on account of, for example, that the frequency in contact between the sleeve and magnetic toner is increased and sufficient triboelectrification of the toner is made possible by very thin application of the magnetic toner onto the sleeve, that agglomeration of the toner particles is avoided and friction between the toner particles and sleeve is made sufficient by supporting the toner with the aid of the magnetic force and causing the magnet and toner to move relatively each other, and that fog is prevented from occurring in the background of the image by supporting the toner with the aid of the magnetic force and opposing the toner to the electrostatic image without any contact therebetween, to effect the development.
These developing methods using a one-component type developer composed of a magnetic toner do not require any carrier. Therefore, it is unnecessary to control the mixing ratio of the carrier and toner. Further, it is not required to conduct agitating operation for mixing sufficiently uniformly the carrier and the toner. These methods are advantageous in that the entire developing apparatus can be made simple and compact.
However, these methods have such drawbacks that the coating layer of the toner on the sleeve is apt to become non-uniform under the conditions of low humidity, and that under the condition of high humidity, particularly high temperature and high humidity, fluidity of the toner decreases and consequently the coating of the toner on the sleeve is apt to become poor and generation of triboelectric charges is liable to become difficult. Further, the moving degree of the toner on the sleeve is restricted. Therefore, differences in the density of the developed image are liable to arise between a portion of the image developed with toner remaining not consumed for the previous development and receiving friction repeatedly, and a portion developed with a toner newly replenished as a result of the toner being consumed for development. Such a phenomenon is observed, for example, in the case in which after many copies of an original having A-4 size is made, another original having a larger width, e.g. B-4 size is copied. That is, the image which is developed with a portion of the toner remaining on the toner-holding member and not having contributed to the copying of the A-4 size original is low in the image density as compared with that developed with a toner newly replenished and present at a portion of the toner-holding member having taken part in copying the same original. The cause of this phenomenon, although not clarified yet, is considered to reside in that the one-component type toner hardly moves in the right and left directions on the toner-holding member, and as a result, in the portion not having been used for development, substantially the same toner always remains to the toner-holding member and is repeatedly rotated in tha state so that new toner is not supplied and exchanged to the corresponding portion of the toner-holding member.
In addition, the developing methods involve a further problem remaining unsolved, so-called ghost phenomenon which is observed in such a case that when a solid black image is formed with a high density and then a new toner is replenished to the sleeve portion hardly having the toner thereon owing to consumption of the toner for the formation of the solid black image, the density of an image developed, immediately thereafter, with the newly replenished toner present at the corresponding portion of the sleeve is reduced.
The developing method using a one-component type insulating toner may be considered to be fundamentally similar to the method utilizing a two-component type developer in such a sense that triboelectric charges are imparted to the toner. In the former developing method, however, it is very important how the toner is uniformly coated to the toner-holding member and such a state is maintained stably. Therefore, the former developing method is largely different from the developing method using a two-components type developer containing carrier particles, also in consideration of the foregoing problems.